gccrs: Fix ICE in path pattern refutability

[u7k4rs6]

Fixes #4732

PathPattern::is_refutable looked up the final path segment's HirId in the HIR item map. A path segment does not carry the HirId of the referenced item, so lookup_hir_item always failed and path patterns in let bindings and function parameters reached rust_internal_error_at.

The fix resolves the path using the path expression's NodeId, then maps the resolved definition through lookup_node_to_hir to find the HIR item. Both Values and Types namespaces are checked.

Enum variants are stored separately from regular HIR items, so failed item lookup falls back to checking the scrutinee type. Non-enum ADTs are also handled to avoid another ICE for unit struct patterns.

This adds the first rust/hir -> rust/resolve include. No build changes are needed since rust/resolve is already on the include path, and there is no include cycle. rust-hir-pattern-abstract.cc already includes rust-hir-type-check.h and calls TypeCheckContext::get () in is_refutable_with_lookup, so this file already depends on a later pass. I can move the lookup into PatternChecker if the direct dependency is considered undesirable, but that would require threading the context through the is_refutable virtual interface since it recurses into nested patterns.

refutable-path.rs now checks the expected diagnostics instead of using dg-ice, covering let bindings, function parameters, an associated constant, and a const generic. irrefutable-path-single-variant.rs was added to directly test single-variant enums and unit structs without hitting the existing backend ICE from #4553.

irrefutable-path.rs is left unchanged and keeps its dg-ice, but now reaches a different ICE. The single-variant enum parameter is correctly considered irrefutable, so compilation proceeds into the backend and hits the pre-existing function_set_parameters assertion from #4553, which irrefutable-slice.rs already XFAILs on.

Validation on the rebased head: 11,598 expected passes, 86 expected failures, 10 unsupported, zero unexpected failures.

[u7k4rs6]

Reworked the hunk and replaced rust_unreachable since non-ADT paths are refutable constants.
Added tests for associated consts and const generics and rebased on master, suite is clean.

[powerboat9]

For

	* rust/compile/refutable-path.rs:
	Remove dg-ice and add associated const and const generic cases.

could you stick more of the second line after the colon?

[u7k4rs6]

@powerboat9 Done with the re- wrapping

[CohenArthur]

The code changes look good to me. I reached out privately for the commit message which I think needs to be changed.